An ideal amplifier has no drift, no voltage offset, no noise, zero settling time and infinite bandwidth. By comparison, an operational amplifier has extremely low drift and offset voltage, but very poor noise, long settling time and low bandwidth. An amplifier made from discrete components has low noise, short settling times and excellent bandwidth but poor voltage offset and drift.
A composite amplifier combines the best traits of the operational amplifier and the discrete, while suppressing the worst traits of both. However, to date there has not been provided a composite amplifier that achieves all of the desirable traits of an ideal amplifier.
Some examples of prior art amplifiers with drift and offset compensation include the amaplifier described by Edwin A. Goldberg, "Stabilization of Wideband DC Amplifiers for Zero and Gain", RCA Review June 1950, in which there is provided a differential amplifier with a circuit used for drift compensation including a mechanical chopper, an AC coupled amplifier, a synchronous rectifier and a low pass filter coupled between the inputs of the differential amplifier. The drift compensation circuit monitors the input of the differential amplifier. If the input drifts by a voltage V.sub.d then this voltage is converted by the chopper into an AC voltage, amplified by the AC amplifier and rectified by the rectifier. The resulting voltage -V.sub.d is then applied to the other of the inputs of the differential amplifier where it tends to cancel the drift V.sub.d. By using an AC amplifier the drift of the AC amplifier is eliminated from the drift compensation circuit. The rectifier and low pass filter smooths the pulsed output of the AC amplifier and yields a DC voltage to be applied to the input of the differential amplifier.
One difficulty with this prior art design is that one side of the differential amplifier adds noise to the input of the amplifier. Another difficulty is that the chopper injects a current into the amplifier's input which has the effect of adding noise to the amplifier.
A similar design to the Goldberg design is shown in Canadian patent no. 750,055 issued to Staunton, in which a drift free amplifier is provided which has a memory device (capacitor) in one of the inputs of the amplifier, and a feedback resistor in the other of the inputs. The memory device is periodically charged from the feedback resistor so that it provides a potential source to offset any drift in the amplifier. The switching of the capacitor also tends to inject noise into the input of the amplifier, as with the Goldberg device. Also, the capacitor output has no gain and therefore the correction voltage will not be as effective as if it were amplified.
More recently, Jim Williams, "Composite Amplifiers Yield High Speed and Low Offset", EDN, Jan. 22, 1987, has described several composite amplifiers. For example, a composite amplifier with drift correction is shown in FIG. 2 of the Williams article in which a differential amplifier is provided with a feedback circuit connected to one of its inputs. The feedback circuit includes a low pass filter and a differential amplifier. The feedback circuit supplies a correction voltage to the other of the inputs of the differential amplifier. Like the second stage of the differential amplifier in the Goldberg design, Q.sub.2 in FIG. 2 of the article tends to add noise to the amplifier. Integrated circuit IC2 also injects unwanted noise into the input of the amplifier.
In another example, shown in FIG. 5 of the Williams article, a composite differential amplifier includes a feedback circuit having an offset gain correcting device LT1008 and a low pass filter. This device suffers from three difficulties. Firstly, whatever gain is selected for the high speed amplifier, the same gain must be matched for the compensation circuit, leading to difficulties in constructing the devices. Secondly, the filters at the inputs of offset gain correcting device must be matched, or settling time may be affected. Thirdly the offset correcting amplifier injects an unwanted noise into the amplifier.
In some prior art amplifiers, the amplifier includes more than one stage and the voltage noise from an offset compensation stage is amplified by the later stages, thus degrading the performance of the amplifier and requiring precise matching of the components of the amplifier.
The present invention provides a composite amplifier that has all the desirable traits of low drift, low offset, short settling time, high bandwidth, high open loop gain, and low noise. The offset and drift correction circuitry increases the open loop gain but does not, to any measurable amount, increase the input noise of the active device, increase the offset current, inject input noise current or inject input noise charge.
Further, this offset and drift correction circuitry can be embodied in an integrated circuit to be used in wide variety of discrete amplifiers. The design of the discrete amplifier can be done independently of the drift and offset correction circuitry so any type of discrete design criteria can be easily achieved.
In one aspect of the invention, therefore, there is provided a discrete active gain element having a feedback circuit that includes an attenuator. In another aspect of the invention, there is provided a discrete active gain element having a feedback circuit including a unique operational amplifier, that is formed from a first differential amplifier and at least a second differential amplifier that has a higher gain than the first. This is believed contrary to teaching in the art in which a later stage has lower gain than an earlier stage to avoid undue addition of noise from the second stage.
These and other features of the invention are described in the detailed description that follows and the claims that follow the detailed description.